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Corrosion

Selection Stainless Steel For Handling Hydrochloric Acid HCl






Hydrochloric acid is classed as reducing acid and lacks the oxidising properties that stainless steel need to maintain their 'passive' corrosion resistant surface layer.

The acid is formed as a product of the pitting / crevice corrosion mechanisms in stainless steel and so when hydrochloric acid is present in any 'external environment' corrosion is promoted. Commercially concentrated acid is around 37wt. %.

The common stainless steel types, 304 and 316 should be considered non-resistant to hydrochloric acid at any concentration and temperature.

Corrosion resistance of stainless steel

The iso-corrosion diagram 0.1mm/year lines are represented for 316 types (red) and a 6% molybdenum austenitic type (green).

Higher grades of stainless steel can have limited resistance, up to around 3% maximum at ambient temperatures, but may suffer local attack mainly as crevice and pitting corrosion, even at such low concentrations. The steep curves for 316 and the higher alloyed grades on the iso-corrosion diagram illustrate their very limited resistance.



(The broken line represents the boiling point)
The iso-corrosion diagram 0.1mm/year lines are represented for 316 types (red) and a 6% molybdenum austenitic type (green).

Any additional chlorides or chlorine in the acid can be expected to make attack more severe. Nickel based alloys, rather than stainless steels, should be considered for handling hydrochloric acid.

Contact between stainless steel and building mortar cleaners

The use of building mortar cleaners that contain hydrochloric acid can result in staining and pitting to nearby stainless steel items. Architectural metalwork and kitchen equipment has been reported with such problems resulting from either splashes or from the fumes given off from the acid.

Commercially available cleaning acids are around 16% hydrochloric acid. Using these either 'neat' or as a one to one (50%) dilution, which may be recommended by manufacturers / suppliers of these cleaners, makes these solutions extremely aggressive to most stainless steel grades.

It is advisable not to use such cleaners anywhere near stainless steel items.

What can be done if building mortar cleaners come into contact with stainless steels?

If damage has occurred it is usually evident as brown staining.
Provided visible pits have not been formed it may be possible to renovate the surface by routine cleaning methods.
If pitting has occurred then this must be bottomed out with abrasives (grinding). This is necessary to avoid further staining or corrosion in these areas.
If this is not acceptable then the affected parts may have to be replaced.

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Releated References:
Selection of Stainless Steel fo Handling Sulphur Dioxide SO2 and Sulphur Trioxide SO3
Selection of Stainless Steel for Handling Phosphoric Acid H3PO4
Selection of Stainless Steel for Handling Hydrofluoric Acid HF
Selection of Stainless Steel for Handling Citric Acid C3H4OH (COOH)3
Selection of Stainless Steel for Handling Ammonia NH3
Selection of Stainless Steel for Handling Chlorine Cl2 and Chlorine Dioxide ClO2
Selection of Stainless Steels For Handling Hydrochloric Acid HCl
Selection of Stainless Steel for Handling Sulphuric Acid H2SO4
Selection Stainless Steel for Handling Sodium Hydroxide NaOH
Selection of stainless steels for handling acetic acid (CH3COOH)
Selection of stainless steels for handling sodium hypochlorite (NaOCl)
Selection of stainless steels for handling nitric acid (HNO3)
NACE MR 0175/ISO 15156 for Corrosion Resistant Alloys for Sulphide Service
Selection of stainless steels in water supply and waste water treatment



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